1. Field of the Invention
The present invention relates to a light-emitting module including a substrate on which a light-emitting device that causes heat generation during lighting such as a light-emitting diode is mounted, and an illuminating apparatus that uses the light-emitting module as a light source.
2. Description of the Related Art
Recently, as a light source for general illumination purpose, a light-emitting module including a plurality of light-emitting devices such as light-emitting diodes have been used. A light-emitting module disclosed in Japanese Patent KOKAI Publication No. 2006-344690 comprises an enamel substrate, on which a plurality of light-emitting devices are mounted. The enamel substrate includes a metal core and an enamel layer that coats the surface of the metal core. The enamel layer is an example of an insulating layer and is formed by baking glass powder, for example, on the surface of the metal core.
The enamel substrate comprises a mount surface and a back surface that is located on the opposite side of the mount surface. A plurality of reflecting cups are formed on the mount surface. The reflecting cups and the mount surface are continuously covered with the enamel layer. A light-emitting device is mounted inside each of the reflecting cups. On the back surface of the enamel substrate, the metal core is exposed outside the enamel substrate after the enamel layer is removed. Thereby, the back surface of the enamel substrate also functions as a heat dissipater.
According to the conventional light-emitting module disclosed in the above-described Japanese Patent KOKAI Publication, dielectric strength between electrical components such as the light-emitting devices and the metal core is secured by the enamel layer. Further, heat gene'rated by the light-emitting device upon lighting of the light-emitting device is transferred from the enamel layer to the metal core. The heat of the light-emitting device transferred to the metal core is dissipated into the outside of the light-emitting module from the heat dissipater of the enamel substrate. It is thereby possible to attain excellent heat dissipation of the light-emitting device, and suppress increase in temperature of the light-emitting device.
The conventional light-emitting module uses only the single enamel layer so as to secure dielectric strength between electrical components such as the light-emitting devices and the metal core. In the process of manufacturing insulating layers such as enamel layers, defects inevitably occur that would deteriorate dielectric strength. In order to obtain a desired dielectric strength when the insulating layer is a single layer, it is necessary to increase thickness of the insulating layer based on the prediction that defects may occur in the insulating layer.
More specifically, the defects that would deteriorate dielectric strength denote voids generated in insulating layers or foreign bodies involved in insulating layers. A defective portion in an insulating layer is no different from a locally thin portion of the insulating layer, and dielectric strength of the defective portion decreases. It is therefore necessary to increase thickness of the insulating layer to compensate for decrease in dielectric strength when the insulating layer is a single layer.
Increase in thickness of the insulating layer, however, inevitably causes increase in thermal resistance of the insulating layer. This interferes with heat transfer from the light-emitting device to the metal core, and results in decrease in heat dissipation of the light-emitting device.
In conventional light-emitting modules, in order to effectively obtain light emitted by light-emitting devices, a single insulating layer is sometimes mixed with white pigments so as to attain improved light reflectivity of the insulating layer.
White pigments, however, are same as foreign bodies to the insulating layer. Accordingly, the insulating layer containing white pigments need to secure a desired dielectric strength by further increasing its thickness. This results in further increase in thermal resistance of the insulating layer and deterioration in heat transfer from the light-emitting device to the metal core.